Theodore Rigg was born at Settle, Yorkshire, England, on 6 April 1888, the son of Hannah Wilson and her husband, John Rigg, a merchant and a staunch Quaker. Two years later he went to New Zealand with his parents, who settled in Newtown, Wellington. He was educated at Newtown School (1894–1902) and at Wellington College (1903–6), where he won several scholarships and science prizes. He joined the Chemical Division of the Department of Agriculture in 1907 and, on winning a Turnbull Scholarship, entered Victoria College as a chemistry student under Professor Thomas Easterfield. A gifted athlete as well as a scholar, he held the New Zealand University three-mile championship for four successive seasons. He graduated MSc with first-class honours in 1911, and was awarded a Jacob Joseph Scholarship for chemical research. An 1851 Exhibition Scholarship took him to St John's College, Cambridge, where in 1914 he was awarded a BA for agricultural research.
The First World War interrupted Rigg's studies. He had followed his father into the Quakers and his abhorrence of the conflict prompted him to look for humanitarian work. After joining a relief organisation of the Society of Friends, he took an active part in distributing food and money to the needy in France, Albania, Montenegro and Russia, at times under conditions of great hardship and danger. He was able to use his organisational and agricultural skills to assist in the farming recovery and the relief of refugees in war-torn countries. While serving with a Quaker unit in the Samara province of Russia in 1917, he met an American, Esther Mary White from Philadelphia, a former teacher. They eventually worked together, maintaining children's colonies, until they left Russia via Finland and Sweden in 1919.
Rigg returned to New Zealand early that year and then went back to Cambridge; he was to be awarded an MA in 1924. He worked under Sir John Russell at Rothamsted Experimental Station, Harpenden, and later in the United States at the Soils Division, Department of Agriculture, and the Soil Department, Cornell University, gaining among other things experience in soil surveying. On 8 October 1919 at Philadelphia he married Esther White. They were to have two children. In 1920, at the age of 31, he joined the foundation staff of the Cawthron Institute, Nelson, under the directorship of Easterfield. This reunion between professor and former student proved of great value to the institute in its pioneering research.
Theodore Rigg entered into Cawthron's research with enthusiasm. By the 1920s serious losses of export apples in cool storage led to an extensive study headed by Rigg in collaboration with the DSIR. The fruit's 'internal breakdown' was found to be influenced by conditions of storage (especially temperature), stage of maturity at picking, type of fruit, growing conditions and soil properties. This knowledge enabled the problem to be successfully managed. Rigg also focused on local problems such as improving the soils of the Moutere hills and the poor pakihi lands through research into fertilisers and trace element deficiencies. Maps from his early soil surveys proved invaluable in later investigations into many problems in plant nutrition and animal health. He then extended the soil and trace element work to other parts of the country, especially that relating to 'bush sickness' and the nutrition of orchards, small fruits, tobacco and hops.
Rigg became a leading figure in all aspects of agricultural research. He became head of the Department of Agriculture and Chemistry in 1924 and assistant director in 1928, and in 1933 (on the retirement of Easterfield) director of the Cawthron Institute. In the early 1930s he was director of the soil reconnaissance survey of the central North Island, and from the mid 1930s he was for nine years officer for the Soil Survey Division in charge of chemical work for the DSIR. He was a founding member of the Council of Scientific and Industrial Research from 1926 to 1954, and became its chairman from 1943. From 1926 he was associated with the foundation and administration of many committees of the council, as well as other organisations concerned with science in agriculture. He was a member of the Nelson Catchment Board from its inception in 1944 and chairman from 1950 to 1956, a member of the New Zealand Advisory Section of the Nuffield Foundation (1946–58), and chairman of the Farm Committee. On two major trips overseas he attended delegations representing New Zealand: in 1927, at an international soil conference in Washington DC, and in 1946 at the Imperial Agricultural Bureaux Conference, London.
During his long working career Theodore Rigg gained many honours and distinctions, both for his practical studies in agriculture and for his lively interest in the broad realm of science, especially chemistry. These included fellowships of the Royal Institute of Chemistry (1925), the New Zealand Institute (1932), and the Royal New Zealand Institute of Horticulture (1947), and honorary doctorates of science from the University of Western Australia (1947) and the University of New Zealand (1957). He was appointed a KBE in 1938. By 1956, when he retired, he had written some 80 of the 700 scientific papers, bulletins, and official reports published by the Cawthron Institute, including results of major soil surveys.
Theodore Rigg had devoted his entire scientific career to the work of the institute. His first wife, Esther, died in 1959 and on 2 December 1966 at Nelson he married Dr Kathleen Maisey Curtis, a retired mycologist and pioneer plant scientist who had spent her working career at the Cawthron Institute. He died in Nelson on 22 October 1972, survived by his wife and two daughters from his first marriage.